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• Before space travel, looking at lightfrom astronomical objects was the only way for astronomers to study them
• Careful study of light lets us measure
• Temperature
• Chemical composition
• Relative speed
• After this lecture, you should be able to answer the questions:
• What is Light?
• What are its Properties?
• How is it Produced?
• What can it Tell us?

• Galileo tried to measure (unsuccessfully) the speed of light (early 1600s)
• Romer measured speed of light using eclipses of Jupiter's moons in 1676
• period of moon's orbits were known
• went behind Jupiter (eclipsed) at regular intervals
• about 6 months later, the eclipses were happening 16.5 minutes late!
• Romer's value can be expressed in minutes per AU since the precise value of 1 AU was not known at the time (what was Romer's number?)
• Fizeau and Foucalt measured the speed of light more precisely in 1850.  Modern value:

• Implications in everyday life:
• Thunder and lightening
• Sound has a speed too, about 0.3 km/s or 600 mi/hr
• Thunder is delayed relative to lightening

 
   

• Newton (1642-1727) used two prisms to show that "normal light" (white light) is really composed of all of the colors of the rainbow
• Because of the way light bent in prisms, Newton thought light was made of tiny particles (now called photons)
• Christiaan Huygens in 1678 argued light is waves.
• NEWTON AND HUYGENS ARE BOTH RIGHT
• Light can be thought of as particles (photons) and waves
• Buzzword: wave packets
• Thomas Young's double slit experiment in 1801 showed wave nature of light
• Electric charge and magnetism were experimented with in the 1700s and 1800s (Ben Franklin and his kite in the thunderstorm)
• James Clerk Maxwell in the 1860s wrote down four equations that described how electricity and magnitism interelate
• A natural consequence of Maxwell's equations is the propagation of electromagnetic radiation
• Maxwell's equations predict the speed of light

• Radiation in the sense of "emanates from"
• An electric field surrounds electric charges
• A magnetic field surrounds magnets
• Magnetic fields make electric charges move
• Moving electric charges make magnetic fields
• Electromagnetic wave is one big happy party governed by Maxwell's Equations
• Electromagnetic waves need matter to start in, but once they get going, they can propagate through nothing at all (vaccuum)

• Wavelength (l=lambda) determines color of light (see next slide)
• Since the speed of light is fixed, you can easily get the frequency, n of light (how many wave crests pass you each second)

• We measure frequency in Hertz (Hz), in honor of Heinrich Hertz who did early work with radio waves

1 Hz = 1 cycle/s

• The color or energy of the light is determined by its frequency, n (think of sneezing).

• h is Planck's constant, in honor of Max Planck

• Recall:

• Therefore:

• Example:  WERN at 88.6 MHz

E = (6.625 x10^-34 J s )(88.6 x 10⁶ 1/s)
E = 5.8 x 10^-26 J
l = (3 x 10⁸ m/s ) / (88.6 x 10⁶ 1/s)
l = 3.39 m

• 'Invisible light' discovered by William Herschel in ~1800 using a thermometer and a prism.  Called it Infrared Radiation.
• Hertz discovered radio waves in 1888 with sparks
• Rontgen discovered X-rays in 1895 because his film kept getting ruined

• Wavelength, l, can vary over a HUGE range
• Visible light only a small portion of the electromagnetic spectrum


4 x 10^-7 m (blue) to 7 x 10^-7 m (red)
or
400 to 700 nanometers (nm)
1 nm = 10^-9 m

• Recall energy, frequency and wavelength are all related
• Short wavelength radiation specified by energy (high energy)
• UV though Infrared refered to by wavelength
• Microwaves through radio refered to by frequency (low energy)



 

• Skip this section in the book
• It is more important when studying stars of different types
• What is a "black body?"
• An object that does not reflect any light
• Black bodies emit electromagnetic radiation (light) that depends on their Temperature
• Example: blacksmith

• Spectral lines were discovered in spectra of the Sun by Joseph von Fraunhofer in 1814
• Materials like salt sparkle certain colors when thrown into flames
• In 1857 Robert Bunsen created a special burner (bunsen Burner) to study these effects
• Fraunhofer (absorption) and emission lines are related!
• Every chemical element has its own unique set of spectral lines
•  What is going on?

• In 1910 Ernst Rutherford beamed alpha particles at thin gold foil
• thin gold foil like tissue paper
• alpha paraticles like bullets
• Some alpha particles bounced back!
• Rutherford concluded that there must be very dense things is the foil somewhere
• The atomic nucleus
• Neils Bohr worked out a model of the Hydrogen atom shortly thereafter
• one proton at the center
• one electron orbiting in certain allowed orbits
• Quantum mechanics predicts the positions of these orbits
• Emission line: electron jumps to lower orbit
• Falling in gives up energy
• Absorption line: electron jumps to a higher orbit
• climbing out takes energy
• Analogus processes in other atoms and molecules

• When something moves toward you, radiation emited from it has an apparently shorter wavelength
• away, longer wavelength

• Dl = change in wavelength
• l = wavelength
• v = relative velocity (speed)
• c = speed of radiation (in this case, light)